What happens if you eat chicken on Good Friday?

A sin is committed. It is a seirous (mortal) sin if the person is above the age of 14 and does not have a special dispensation, knows that it was a sin to do so, and ate meat anyways. It is a venial sin if it was done by accident or without really understanding this or was of course, not consenting to the act through free will.

In the case of a mortal sin, a confession is needed. In the case of venial sin, a confession is a good idea, but not required. A normal act of contrition and some self-specified penance is needed.
It can happen, it is a sin if the person is 14 years or older and does not have a dispensation. If it was done because one was careless then the sin is still a grave one. Seek a confession in this case.